Project Summary Failure to resolve chronic infections is a significant worldwide health concern. A major roadblock to resolution of these infections is the gradual loss of effector functions within responding T cells. This process, referred to as T cell exhaustion, occurs in many cancers as well, making the need to understand the biological mechanisms that drive development and maintenance of this population even more pressing. Many studies have shown that boosting the immune system or reinvigorating exhausted T cells can help control chronic infections and successfully treat an increasing number of cancers. However, most strategies are either not effective or not durable. The field now appreciates the complexity of this diverse TEX cell population and is finding that manipulation of both the differentiation status and the chromatin state of TEX cells may be required to establish effective and durable reinvigoration of these cells. The long-term goal is to define the mechanisms underlying T cell exhaustion. The goal of this proposal is to investigate the role of the methylcytosine dioxygenase, TET2, in regulating this differentiation process. Loss of TET2 promotes CD8 memory T cell formation in both murine and human T cells as well as regulating the DNA methylation status of these cells. Given the role of TET2 in directing both cellular differentiation and the epigenetic landscape, we hypothesize that TET2 regulates the development of exhausted CD8 T cell subsets. This proposal seeks to address this hypothesis in the following two aims: AIM 1: Determine the role of TET2 in TEX cell differentiation, and AIM 2: Determine the ability of TET2 inactivation to enhance the efficacy of PD-1 pathway blockade during chronic viral infection. These studies have the potential to uncover new mechanisms and therapeutic targets of exhausted T cells.